This invention relates to a novel method for evaluating the vestibular system, i.e., those parts of a living body concerned with the maintenance of equilibrium. More particularly, it relates to a method of this general character in which electrical stimulation is employed in a simple and highly effective manner to evoke responses indicative of the condition of the vestibular system.
Computer averaging of on-going micro-electrical activity in the central nervous system has provided a tremendous growth in the field of physiologic evaluation of neurologic systems. The measurement of auditory function, through the use of surface electrodes, has remarkably improved the diagnostic and rehabilitative capabilities of the medical, audiologic and rehabilitation communities in the past fifteen years. As a result, there are a great number of commercially available clinical signal averaging instruments for evoked response measurements in use at this time. Most of these have a multi-modal stimulus capability and are supplemented by programmable and/or dedicated computers and data storage facilities.
One of the sensory pathways, however, the vestibular system, has been relatively neglected due to the lack of a reliable and less traumatic vestibular stimulation apparatus than has been available in the past. The current technique of evaluation of the vestibular system involves, in part, either the irrigation of the ears with cold and hot water for a period of time or moving the patient in a chair. These types of stimulation generate responses which are measured by the resulting reflexive eye movements. The procedures allow for a wide range of errors, cross at least two separate systems (optic/vestibular), are physically uncomfortable and require a degree of patient cooperation.
It is an object of the invention, accordingly, to provide a new and improved method for evaluating the vestibular system that is accurate and reliable, yet can be practiced with minimum discomfort to a patient being tested.
According to the invention, vestibular stimulation is effected by applying to the mastoid area of a patient metal electrodes covered with an insulating material such as Mylar, for example, connected in a series resonant circuit including the electrodes and the body tissues therebetween, energizing the series resonant circuit by an ultrasonic frequency carrier, the frequency of which is determined by the series resonant circuit, and modulating the amplitude of the carrier by a frequency in the range from about 0.1 Hz to about 15 Hz. The applied carrier appears to produce vibrations in the tissues at the frequency of the modulating signal that are essentially received as bone-conduction signals. Those signals evoke vestibular responses similar to those produced in the conventional irrigation or moving chair techniques, i.e., the patient has the same feeling of imbalance but avoids the unpleasantness of the first technique and the cumbersomeness of the second .